Heat insulators for use in constructing, for example, walls of a heat treating baking oven which require a high degree of heat insulation are known, as described in, for example, Japanese Patent Publication No. 60-8399 which teaches a heat insulator comprising a pair of spaced outer skin materials having an enclosed space defined therebetween, with an inorganic foamed powder material packed highly densely within the enclosed space, the interior of the enclosed space being vacuumized.
Heat insulators of this type exhibit high heat insulation performance through combined effects of both the vacuum atmosphere present within the space and the thermal insulation effect of the inorganic foamed powder mass or fibers densely packed in the space. With such a heat insulator having, for example, a wall thickness on the order of 4 to 5 cm, heat insulation is possible between ordinary temperature and a temperature having a temperature difference range 0f 300.degree. to 400.degree. C. relative to the ordinary temperature.
However, such a known heat insulator involves a problem that while any heat that will otherwise pass through the thermally insulated space by way of heat conduction can be sufficiently reduced, passage of heat by radiation cannot sufficiently be reduced. From the standpoint of effective heat insulation, therefore, there still exists a room for further improvement.